Axially reciprocating displacers in power conversion machinery, such as Stirling cycle machines, have been supported in housings to form clearance seals between portions of a reciprocating displacer and the housing. A typical displacer forms a piston that is movably carried within the housing. Reciprocating movement of the displacer within a chamber of the housing transfers working fluid between the front and back sides of the displacer, causing a thermodynamic transformation there between. Movement of the displacer occurs between a compression space, having a temperature somewhat above ambient, and an expansion space, having a low temperature (when configured in a cooler) or high temperature (when configured in an engine).
In one case, for a Stirling cryocooler, an end portion of a reciprocating displacer forms a drive area in fluid contact with the compression space. The displacer end portion slidably extends through a bore in the housing in fluid communication with a compression space of a linear drive motor. The drive motor has a driving piston that operates on working gas in the compression chamber. The working gas then directly works on the displacer to produce motion. Hence, the driving piston and displacer form a free piston machine, cooperating solely by action of the working fluid. A clearance seal is typically provided between the displacer end portion and the housing bore by maintaining an accurate reciprocating motion of the displacer and by providing an accurate relative sizing of the bore in the housing with the working piston and displacer end portion. The expansion space draws heat from a surrounding cold head, imparting cooling there along. The same construction can form a Stirling engine, by simply imparting heat to the cold head, causing the displacer to reciprocate, and moving the linear drive motor (which now acts as a linear alternator) to produce electric power.
Techniques previously identified for supporting a displacer in a sprung configuration within a housing chamber in power conversion machinery include 1) flexural bearings used to accurately position a reciprocating member in a housing with respect to a clearance seal, and 2) gas spring/bearing supports/seals used to spring a free piston displacer.
In Stirling cycle machines having movable free piston displacers, such as engines and coolers, the displacer has to operate in a near resonant condition. Otherwise, the displacer can move out of phase with the alternator/motor piston, causing a serious or inoperative condition. One previously utilized technique for springing a displacer involves the use of a gas spring on a free piston displacer. For cases where the displacer is sprung to ground via a gas spring, gas forces on the displacer rod provide only a portion of the spring effect required for a displacer that is sprung to ground. The resulting amount of spring is generally only a small fraction of the spring necessary to impart a resonant condition for a displacer of typical size and mass.
For Stirling cycle machines having piston displacers sprung to ground with flexure bearings, the flexure bearings provide an axial spring effect as well as a non-contact operation of the displacer and associated rod seal. Generally, the sum total of axial spring forces produced from each flexure of an assembly is frequently not sufficient to provide a spring force sufficient to impart near resonant conditions. Therefore, it becomes necessary to add additional flexures. Depending on the specific flexure design and mounting technique being employed, the additional number of flexures that are needed can be significant. In some cases, particularly at high operating frequencies and/or with relatively large and heavy displacers, it may be physically impossible to provide the number of flexures that are required to realize the required resonant condition. Adding extra spring flexures increases the moving mass, which necessitates adding additional springs to achieve a desired spring/mass ratio. However, for conditions where a high spring/mass ratio is necessary (i.e. high frequencies), the moving mass of the flexure may prevent one from ever realizing the desired spring/mass ratio.
Therefore, there is a need to provide an improved displacer spring construction that makes it physically possible to realize near resonant operating frequencies, particularly at high operating frequencies and/or for heavy displacer masses. The present invention also arose from an effort to develop such an improved construction in a simplified, economical, and cost effective manner.